Keeping moisture out of a closed crawl during construction
Was just reading Carl’s article on closed crawlspaces butdecided to post in Q&A.
We too have seen some issues with closed crawls, mostly on retrofits of existing crawls that have been done improperly by others . Saw one the other day where the crawl was closed in but no ventilation (supply air, etc.) was provided. The entire floor system rotted so badly you could break the joist with your hands. I think if conditions are not perfect, in our climate (NC 3A), the conditions can worsen more quickly in a closed crawl as opposed to a vented crawl where most get wet during the summer and then dry out during the winter. This has not turned us away from closed crawlspaces, we still install them and recommend them, but stress the importance of monitoring these spaces to our home owners. Due to all of the issues we have seen, we now only use the exhaust only approach and install a dehuemidifier as a back up if our exhaust fan breaks or does not get the job done. There is almost always some sort of mold in the crawl somewhere no matter what is done. Which brings me to my question – any ideas on how to control the moisture during construction? We install our crawl doors and a sacrificial vapor barrier right away and install a dehumidifier that runs during the entire project but only does so much good. We have even gone through the trouble of spraying the entire floor system with an ant-mold product but still had some minimal mold growth during the construction process. Anyone come up with any better ideas to control moisture in the humid south during construction?
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Replies
Danny,
sorry I don't have an answer but I do have a question for you. You say
"Due to all of the issues we have seen, we now only use the exhaust only approach and install a dehuemidifier as a back up if our exhaust fan breaks or does not get the job done."
I'm curious why the exhaust-only approach. I've seen schematics for passive transfer grates supplying dry air (presumably) from the conditioned space with an exhaust fan at the opposite end of the crawl.
Also, in NC are you using the "termite inspection strip" requirement we have in Z 3A (GA)? A 3-6" gap in the insulation at the top of the foundation might have a real impact ... tho I realize that work probably wouldn't be done until post-construction in any case.
Thanks for bringing this up.
Joe - yes we do need to leave a 3" termite inspection gap when we install insulation on the exterior foundation wall. we have been thinking about making a removable piece or pehaps a piece that folds down for inspection purposes so the gap is not there year round but have not run this by the local building standards yet.
Our reasons for exhaust only approach:
The majority of what we do is retrofit existing crawlspaces with moisture problems. These crawlspaces are filled with mold, insects and a bunch of other wet, dirty smelly stuff. The traditonal supply duct to condition the crawl would require removing all the existing insulation, removing all the mold and a full cleaning of the crawlspace since once you pressurize the crawl, you are essentially mixing the crawlspace air with the indoor air. Most of our customers do not want to spend $10K+ on their crawlspace. The exhaust only approach will exhaust all of these contaminants and bring the clean indoor air down into the crawl. In certain situations on tight budgets, we will even leave the insulation at the floor level and just close the vents, air seal the foundation, install a full vapor barrier over the ground and add our exhaust fan. A "closed crawl" not a "conditioned crawl" but manages the moisture just as well. (We stress the importance in these situations that the crawl must be monitored regularly to make sure this is working and additional measures may be needed down the road. We will leave a $100 datalogger in the crawl and inspect on a regular basisi)
Other benefits of the exhaust only approach - although not a recommended fix for radon remediation, it can reduce radon levels in a home, it can help satisfy ASHRAE 62.2 fresh air ventilation requirements and lowers the neutral pressure plane of the house reducing stack effect during the heating season.
On our new construction jobs, we used to use the supply duct approach until recently. After seeing how quickly closed crawls can deteriorate when not done properly, it made sense to us to use the exhaust only for both new and exisintg crawls;
A. Cannot prevent moisture accumlation during the construction process and if there is mold in the crawl, do not want to spread it through the house and
B. If there ever is a future problem, water pipe leak, etc - this air will once again not make it into the house.
I know some experts disagree with this approach but it has been working well for us in our climate.
Hope that helps some.
We've not experienced these problems (Zone 4A, NC Triangle area). In retrofit situations it's essential to dry out the crawl with proper exterior grading and waterproofing before doing any sealing or insulation. In new construction we leave the access door open until the house is dried in. In certain extremely wet and humid summer conditions we can get some mold appearing on framing throughout the house, no more in the crawl space than elsewhere. This is treated with bleach before close-in.